LAKES AND TARNS 143 



could be rapidly regrown. So he called it Hydra, for 

 it was characteristic of the monster that when Her- 

 cules slashed off a part it was at once replaced by 

 another. Trembley was greatly pleased with his dis- 

 covery, which he studied thoroughly for several years 

 and made the subject of a very interesting book. 



The Hydra hangs from the under side of the Duck- 

 weed disc. When everything is still it lengthens 

 itself and sways its threadlike tentacles gently in the 

 water. They touch small animals and poisonous 

 lassos are exploded from stinging cells. The small 

 animal is gripped and paralysed, and then the tentacle 

 tucks it into the Hydra's mouth. Sometimes the 

 Hydra shifts its position on the Duckweed without 

 letting go; sometimes it loops along, fixing mouth 

 and base alternately in leechlike fashion; sometimes 

 it leaves the Duckweed altogether and writhes gently 

 in the water. When food is abundant it produces 

 buds, and these may bear more buds; when food is 

 scarce the buds are separated off, and thus the popula- 

 tion of polyps increases. But it also multiplies by 

 fertilised egg-cells, only one being produced at a time. 



One of the common kinds of Hydra has a brown 

 pigment Hydra fusca; another is greyish; another is 

 green Hydra viridis; and it seems that the green 

 colour is due to microscopic unicellular Algae which 

 live inside the inner layer cells of the body in an 

 intimate, mutually beneficial partnership which is 

 called " symbiosis." In the sunlight the microscopic 

 green partners are able, like the green cells in the 

 leaves of plants, to split up carbonic acid gas produced 

 by the cells of the Hydra, to liberate oxygen, which 

 is all to the good for the Hydra, and to build up some. 



